In some applications, in particular the steering systems for the rear wheels of a four-wheel steering vehicle, electric steering actuators are favored because of their desirable power characteristics and compact and light-weight design. For instance, the electric steering actuator disclosed in U.S. Pat. No. 5,330,023 comprises an electric motor having a hollow output shaft, a steering shaft passed through the hollow center of the output shaft of the electric motor, and a ball-screw mechanism provided between the output shaft and the steering shaft for converting the rotary movement of the output shaft into a longitudinal movement of the steering shaft. The ball-screw mechanism comprises a threaded section of the steering shaft, and a nut member which threadably engages with the threaded section of the steering shaft via steel balls, and is secured to the output shaft of the electric motor via a flange coupling.
According to this previous proposal, because threaded bolts have to be passed through a first flange, and threaded into holes provided in a second flange, the axial length of this coupling structure tends to be significant. Furthermore, because the rotor of the motor must be joined to the nut member before the entire assembly is placed in a casing, a substantial amount of work is required for the assembling process. Also, some difficulty is involved in servicing the actuator, and this adds to the cost for the maintenance.